(a) Field of the Invention
The present invention relates to a display device and a driving method thereof, and in particular, a light emitting display device and a driving method thereof.
(b) Description of Related Art
The flat panel displays include a liquid crystal display (LCD), field emission display (FED), organic light emitting diode (OLED) display, plasma display panel (PDP), and so on.
Generally, an active matrix flat panel display includes a plurality of pixels arranged in a matrix and displays images by controlling the luminance of the pixels based on given luminance information. An OLED display is a self-emissive display device that displays image by electrically exciting light emitting organic material, and it has low power consumption, wide viewing angle, and fast response time, thereby being advantageous for displaying motion images.
A pixel of an OLED display includes an OLED and a driving thin film transistor (TFT). The OLED emits light having an intensity depending on the current driven by the driving TFT, which in turn depends on the threshold voltage of the driving TFT and the voltage between gate and source of the driving TFT.
The TFT includes polysilicon or amorphous silicon. A polysilicon TFT has several advantages, but it also has disadvantages such as the complexity of manufacturing polysilicon, thereby increasing the manufacturing cost. In addition, it is hard to make an OLED display employing polysilicon TFTs be large.
On the contrary, an amorphous silicon TFT is easily applicable to a large OLED display and manufactured by less number of process steps than the polysilicon TFT. However, the threshold voltage of the amorphous silicon TFT shifts as time goes due to a long-time application of a unidirectional voltage to a gate of the TFT such that the current flowing in the OLED is non-uniform to degrade image quality and the lifetime of the OLED is shortened.